Why do you feel the coffee get hotter when you stir it?
That little surprise is the world of heat, temperature and thermal expansion – the three buddies that make everyday objects change their feel and size.
💡 In Simple Words: Heat is energy that moves from a hotter object to a cooler one. Temperature tells us how hot or cold something is. When things get hotter, they usually stretch a bit – that’s thermal expansion.
What is Heat?
Heat is the form of energy that flows because of a temperature difference. Think of it like water flowing from a high hill to a low valley; the hill is the hot side, the valley the cold side, and the water is the heat moving.
- It is not a property stored in a body – it is the transfer itself.
- Measured in joules (J), the same unit we use for any kind of energy.
What is Temperature?
Temperature measures how fast the tiny particles (atoms or molecules) inside a material are jigging around. Faster jiggle = higher temperature.
- It is a property of the material, like its colour or mass.
- Common scales: Celsius (°C), Kelvin (K) and Fahrenheit (°F).
Heat vs Temperature – Quick Comparison
| Aspect | Heat | Temperature |
|---|---|---|
| What it measures | Energy transferred due to temperature difference | Degree of hotness or coldness of a body |
| Unit | Joule (J) | Celsius (°C), Kelvin (K) |
| Nature | Transfer (process) | State (property) |
| How we feel it | Through warming or cooling of our skin | By reading a thermometer |
Thermal Expansion – When Things Grow Bigger
Most solids, liquids and gases get a tiny bit bigger when their temperature rises. This change in size is called thermal expansion. Imagine a metal railway track on a hot summer day – it expands a little, and if the engineers didn’t leave a small gap, the track could buckle.
Why does it happen?
When particles jiggle faster (higher temperature), they need a little more space. The average distance between neighbouring particles increases, so the whole object stretches.
Types of Thermal Expansion
- Linear expansion: Change in length of a rod or bar. Formula: ΔL = α L₀ ΔT, where ΔL is the change in length, L₀ is the original length, ΔT is the temperature change, and α is the coefficient of linear expansion (a material‑specific number).
- Area expansion: Change in surface area of a plate. ΔA = 2α A₀ ΔT (approximately).
- Volume expansion: Change in volume of a liquid or solid. ΔV = β V₀ ΔT, where β is the coefficient of volume expansion (≈3α for most solids).
Worked Example – How much does a steel ruler grow?
Suppose a steel ruler is 30 cm long at 20°C. Its coefficient of linear expansion α = 12 × 10⁻⁶ /°C. Find its length at 80°C.
Step 1: Find temperature change, ΔT = 80°C – 20°C = 60°C.
Step 2: Use the formula ΔL = α L₀ ΔT.
ΔL = 12 × 10⁻⁶ × 30 cm × 60 = 12 × 10⁻⁶ × 1800 = 0.0216 cm.
Step 3: Add the change to the original length.
New length = 30 cm + 0.0216 cm ≈ 30.022 cm. So the ruler gets about 0.02 cm longer – tiny, but enough for precision instruments.
Everyday Examples of Thermal Expansion
- Metal bridges have expansion joints – tiny gaps that let the metal stretch in summer and shrink in winter.
- Glass bottles sometimes burst if you fill them with hot water and then cool them quickly.
- Thermometers work because the liquid (usually mercury) expands inside a thin glass tube.
Key Points to Remember
- Heat flows from hot to cold until temperatures equalise.
- Temperature is a measure of average kinetic energy of particles.
- Thermal expansion is linear for small temperature changes; use the appropriate coefficient.
- Always consider expansion in engineering – ignore it and structures can fail.
📝 Likely Exam Questions
- Define heat and temperature. How are they different?
Heat is energy transferred because of a temperature difference; temperature is a measure of how hot or cold a body is (average kinetic energy of its particles). - Write the formula for linear expansion and explain each term.
ΔL = α L₀ ΔT, where ΔL is the change in length, α is the coefficient of linear expansion (material‑specific), L₀ is the original length, and ΔT is the temperature change. - A copper wire 2.0 m long at 20°C expands by how much when heated to 70°C? (α for copper = 17 × 10⁻⁶ /°C)
ΔT = 50°C, ΔL = 17×10⁻⁶ × 2.0 × 50 = 0.0017 m = 1.7 mm. - Why are expansion joints provided in railway tracks?
They accommodate the small increase in length of the rails due to thermal expansion, preventing buckling or derailment. - Explain why a thermometer works.
The liquid inside (mercury or coloured alcohol) expands uniformly with temperature. Its rise in a narrow glass tube directly shows the temperature change.